1. Field of the Invention
The invention relates to the field of magnetic materials and more particularly composites comprised of epitaxial films possessing magnetically switchable states of magneto-optic rotation. The films are supported on monocrystalline non-magnetic garnet substrates.
2. Description of the Prior Art
Epitaxial garnet films of numerous compositions have been developed in the art which possess magnetic and/or magnetic-optic characteristics variously suitable for "bubble devices," magneto-optic detection and/or display devices.
The epitaxial films are generally grown on gadolinium gallium garnet (GGG) or other garnet substrates.
It is impossible to generalize and attribute properties to the composition of all such epitaxial films because many variables interplay in the achievement of desired magnetic/magneto-optic properties.
The variations observed in the formulae of epitaxial film layers do not require elaboration. The art is replete with examples of dramatic and unexpected changes achieved where ionic substitution is effected in epitaxial garnet films and/or where the quantity of one or more components of such films is varied.
Of utmost importance in epitaxial garnet film magnetic devices is the capability of replicating materials which have consistent magnetic properties.
One area where magnetic epitaxial films have found use is the bubble memory. Indeed, epitaxial films useful in the magnetic bubble area have been highly developed.
In bubble films, micron sized magnetic domains are propagated within the volume of contiguous garnet films. The bubbles are actually tiny islands of opposite polarization in the magnetic film layer. Patterns for the bubbles in the form of tiny chevrons define pathways and resting places for the bubbles and a rotating magnetic field causes the bubbles to move from chevron to chevron, thereby storing and retrieving digital data.
Such bubble materials have been formulated from bismuth-containing and rate earth ion containing single crystal strain-free garnet films on lattice matched non-magnetic garnet substrates wherein elevated levels of bismuth incorporation have increased Faraday rotation of the film material and, therefore, provided for optical detection of bubbles. See U.S. Pat. No. 4,018,692.
By reference to U.S. Pat. No. 3,886,533, among others, it is observed that in "bubble devices", domains have been achieved using two major approaches. The first approach involves growth induced anisotropy wherein a mixed population of selected ions at a given crystallographic site results in local strain and/or preferential ordering. The second approach involves the use of strain induced anisotropy, induced by a differential between lattice dimensions of the epitaxial layer and its substrate. Patentees note the advantages and disadvantages associated with each approach pointing out that the latter approach has the disadvantage of significant temperature dependence of magnetic properties of concern in device operation. Patentees also refer to the other and overwhelming problem associated with the existence of a lattice differential, namely, the occurrence of film cracking. The magnitude of the strain induced effect produced by a lattice differential is limited to prevent cracking. Indeed, the matching of epitaxial layer and substrate lattice dimensions is generally specified in the prior art in order to avoid cracking.
Regarding the matter of lattice matching, it must be added that the prior art also includes disclosure of selected epitaxial layer "bubble film" compositions, which in part, achieve their utility by virtue of lattice differential between epitaxial layer and substrate. Representative patents are: U.S. Pat. Nos. 3,886,533; 3,995,093; 4,002,803; 4,263,374; 4,138,530 and 4,169,189.
Finally, before leaving the "bubble film" area, it should be pointed out that yet another method of affecting the magnetic anisotropy of magnetic bubble film garnet materials involves ion implantation. See, e.g., U.S. Pat. No. 3,792,452 and the citations therein.
Epitaxial garnet films have also been used in switchable magneto-optic devices of the type for which the composites of the present invention are suitable. For example, in an article entitled FAST SWITCHABLE MAGNETO-OPTIC MEMORY-DISPLAY COMPONENTS, B., Hill et al., Philips J. Res. 33, 211-215, 1975, wherein magneto-optic iron-garnet films allowing fast switching of a pattern of domains are disclosed, switching is achieved by setting up temperature gradients; therefore, temperature sensitivity is a prerequisite of operation and hence stability of the magnetic properties of such a film is not desirable at the switching temperature of the Hill et al. device.
The present invention pertains to epitaxial films which are useable in magnetically switchabe magneto-optic devices similar to those disclosed by B. Hill et al. in their above-identified article entitled FAST SWITCHABLE MAGNETO-OPTIC MEMORY-DISPLAY COMPONENTS but having the advantage over the Hill et al. devices of being switchable without the need of setting up temperature gradients.
The epitaxial films of the present invention do not require temperature compensation or biasing magnetic fields for domain stability or switching. Rather the epitaxial films of the present invention represent a specific group of magnetic garnet materials possessing uniaxial anisotropy and meeting the condition that H.sub.k -4.sub..pi. M.sub.s &gt;H.sub.sat.
The stability and non-thermal switching capability of epitaxial films meeting the conditions of H.sub.k -4.sub..pi. M.sub.s &gt;H.sub.sat was first disclosed by Pulliam et al. in an article entitled Large Stable Magnetic Domains, J. App. Phys. 53(3), 2754-8 1982.
The present invention relates to the discovery of a specific group of garnet materials, which meet the conditions H.sub.k -4.sub..pi. M.sub.s &gt;H.sub.sat and which are particularly suited for the production of switching elements and arrays of pixels suitable for use in a variety of magneto-optic devices.